The present invention relates generally to load lifting and lowering devices and, more particularly, to a lift having a platform which remains substantially level during raising and lowering but which is automatically tiltable at ground level to define a ramp for easily wheeling a load onto and off of a planar top surface of the platform.
A common type of load elevator or lift comprises a substantially rectangular platform connected at one edge to the swingable ends of a laterally spaced apart pair of parallelogram linkage systems. The other ends of the linkage systems are pivotally connected to a support such as the chassis of a truck or a loading dock and a power means, usually hydraulic, is interconnected to the pair of linkage systems for moving the platform between ground level and that of the truck or dock. Each of the linkage systems comprises an upper arm which operates in tension and a lower arm, referred to as a lifting arm, which is subject to compression and is therefore relatively massive. The power means, typically one or two hydraulic cylinders, is drivingly interconnected to the pair of lower arms and is generally oriented to operate in parallel to the lower arms. The swingable rear ends of the linkage systems comprise a vertical link in the form of a shackle against which the weight of the platform and the load thereon are imposed.
As adapted for use on a truck these lifts may have the platform pivotally interconnected to the upper pivot pins of the rear shackles of the parallelogram linkage systems. In a so-called conventional liftgate the load platform may then also serve as a vertically standing tailgate to close the rear end of the truck bed during transit. In a fold under liftgate, the platform is usually of a two piece construction and both pieces are invertible with respect to the parallelogram linkage systems in order to be moved to an out of the way position beneath the bed of the vehicle during transit. In either case, the linkage systems may be adapted to give either a level ride, in which the platform remains substantially level during raising and lowering, or a ramping action in which the load platform gradually and slightly changes its inclination relative to a horizontal plane during raising and lowering. In both cases, as the platform is connected to the upper pivot points of the linkage systems, a short ramp section is included in the rear edge of the platform assembly to extend between ground level and the planar load bearing area of the platform to facilitate wheeling a load onto and off of the platform.
U.S. Pat. No. 4,002,106 discloses a conventional liftgate whose load platform remains substantially horizontal during raising and lowering and which at ground level is capable of tilting the platform to bring its rear edge into engagement with the ground thus providing a ramp. However, the device is relatively complex in that it employs two pairs of hydraulic cylinders, one of which provides the power to raise and lower the platform. The other relatively complex pair of cylinders provides the power for a tilting action of the platform.
It is also known in the prior art to devise a fold under liftgate having a platform which remains substantially horizontal during raising and lowering and which at ground level operates to tilt the platform to bring its rear edge into contact with the ground. In this device each of the parallogram linkage systems has a lower or compression arm which is made up of two pivotally interconnected knuckle joint sections and is interconnected within the linkage such that the extreme opposite ends of the two interconnected sections are foreshortenable to induce the tilting action of the platform. This device has not proved satisfactory in use in that the knuckle joints have been known to inadvertently open up to induce an undesired tilting of the platform during raising and lowering.